1. Field of the Invention
The invention is in the field of electronic filters. More specifically, the invention relates to an integrated low-pass filter for damping relatively high-frequency components of a signal (e.g. harmonics).
Multipolar single-stage filters are difficult to implement in integrated bipolar high-frequency circuits. One essential goal is a steep edge dropoff in the amplitude response of the filter toward high frequencies, but without having to have recourse to multistage filter arrays.
In the past, the art has either dispensed with explicit filter action entirely, or else used unipolar filters in the form of RC circuits.
2. Summary of the Invention
It is accordingly an object of the invention to provide an intergrated low-pass filter, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and that has a high selectivity in the depletion range, and an optimized characteristic curve with minimal damping in the conducting range, and that can be integrated in an integrated circuit.
With the foregoing and other objects in view there is provided, in accordance with the invention, an integrated low-pass filter, comprising:
an input terminal, a transistor having a control terminal and an output circuit, and a resistor connected between the input terminal and the control terminal of the transistor; PA1 a first capacitor connected between the resistor and the control terminal of the transistor, on the one hand, and to the output circuit of the transistor, on the other hand; PA1 an output terminal connected to the output circuit of the transistor; PA1 a second capacitor connected between the output terminal and a first reference potential; and a current source connected (in parallel with the second capacitor) between the output terminal and the first reference potential; and PA1 the output circuit of the transistor being connected between a second reference potential and, via the current source and the second capacitor the first reference potential.
In accordance with an added feature of the invention, the current source is an adjustable current source. This allows adjusting the operating point of the filter. In addition, the filter can be used as a dual band filter; that is, a switchover between two different frequency bands with correspondingly different filtering is possible. Because of the adjustable current source, the limit frequency (corner frequency) of the low-pass filter is variable, thus making it possible to process a plurality of different fundamental frequencies with a desired constant harmonic damping.
In accordance with an additional feature of the invention, the resistor is a parasitic component of the transistor. This allows a considerable savings in the required chip surface area. Similarly, in accordance with another feature of the invention, the first capacitor is a parasitic component of the transistor.
In accordance with a further feature of the invention, the second capacitor is a parasitic component of the transistor.
In accordance with again an added feature of the invention, the transistor is an npn transistor. Similarly, the transistor may be a pnp transistor that is correspondingly integrated. Alternatively, the transistor is a field effect transistor.
In accordance with a concomitant feature of the invention, there is provided a low pass filter assembly which is formed with a plurality of low-pass filters. The individual filters are connected in cascade fashion, and a buffer amplifier is connected in between and separates the respective low-pass filters.
The invention has the advantage that better selectivity can be attained than when there are two or more cascaded RC members. In addition, unlike with RC members, the useful signal is damped only minimally.
Another advantage of the invention is that especially at intermediate frequencies (100 to 200 MHz), the circuit requires less chip area than conventional embodiments, because when inductances are used they have to be made very large.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an integrated low-pass filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.